1. Field of the Invention
The present invention relates to a method for improving fiber-containing cement mortar and concrete, characterized in that a salt of a .beta.-naphthalenesulfonic acid-formaldehyde high condensate (hereinafter referred to as "NSF") is added as a cement dispersant or water-reducing agent for cement when the ingredients of a fiber-containing cement mortar or concrete are mixed, thereby to improve the workability of the fresh fiber-containing cement mortar or concrete and to improve the physical properties of the resulting cement mortar or concrete.
2. Description of Prior Arts
Civil engineering construction materials including cement have very excellent properties, but they are still insufficient in various points. Various research works have been made on the incorporation of metal fibers, glass fibers and processed natural and synthetic fibers in mortar or concrete so as to overcome some of these defects, for example, poor tensile strength, poor flexural strength, insufficient deformability, insufficient impact resistance and poor crack resistance. Practically, however, it is difficult to attain sufficient fiber-reinforcing effects on the physical properties of a mortar or concrete obtained using a selected cement or concrete composition, simply by incorporating the fibers under the same mixing conditions as those adopted for mixing the ingredients for a fiber-free cement mortar or concrete. Namely, if mixing conditions considered to be appropriate for ordinary fiber-free mortar or concrete are adopted in preparing fiber-containing mortar or concrete and the fibers are merely incorporated under such mixing conditions, the workability is drastically degraded. More specifically, while fiber-containing mortar or concrete is being mixed, horse dung-like fiber masses called "fiber balls" are formed locally here and there in the mortar or concrete. If such fiber balls are present, the workability of the fresh mortar or concrete is extremely lowered and filling or placing of the mortar or concrete into a molding form becomes impossible, and even if such mortar or concrete is forcibly filled or placed, the incorporated fibers are not uniformly dispersed in the mortar or concrete and the expected reinforcing effects cannot be attained. p As means for overcoming the difficulties caused by a reduction of the workability when fibers are incorporated into mortar or concrete, there have been adopted a method in which mechanical means is employed and a method in which the composition of the ingredients of the mortar or concrete is changed. According to the former method, for example, spraying using a sprayer or the like is conducted at the forming step, or a special mixer or fiber dispenser is used at the mixing step. According to the latter method, for example, the proportion of fine aggregate is increased, round gravel is used as a coarse aggregate, the maximum size of the coarse aggregate is diminished, the unit amount of cement is increased, fly ash is added, or the water-cement ratio is increased. Even if the foregoing methods are adopted, however, no satisfactory workability-improving effect can be obtained. Further, when the composition is changed, some of the inherent properties of the mortar or concrete are lost and the application fields are inevitably limited. Furthermore, as will be apparent from the Examples given hereinafter, when fibers are not incorporated, fresh concrete which can easily be fed by a pump under a predetermined consistency can be obtained. However, if fibers are incorporated, this pressure feeding by the pump becomes very difficult and the consistency of the fresh concrete is lowered. If water is further added to this composition (namely, the unit amount of water is increased and the water-cement ratio is made higher), the consistency is restored to the original good state of the fresh concrete having good workability. However, in this case, mechanical characteristics, such as flexural strength after 28 days and compression strength after 28 days, are not improved by incorporation of the fibers in comparison with those of fiber-free concrete.